Photolithography is commonly used to make miniaturized electronic components such as integrated circuits in semiconductor manufacturing. In a photolithography process, a layer of photoresist is deposited on a substrate, such as a silicon wafer. The substrate is baked to remove any solvent remained in the photoresist layer. The photoresist is then selectively exposed through a photomask with a desired pattern to a source of actinic radiation. The radiation exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer solution to remove either the exposed portions of the photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The patterned photoresist can then be used as a mask for subsequent fabrication processes on the substrate, such as deposition, etching, or ion implantation processes.
Advances in semiconductor device performance have typically been accomplished through a decrease in semiconductor device dimensions. The demand for ever smaller semiconductor device calls for higher exposure resolution and better depth of images in photolithography processes. Attenuated phase shift masks (PSMs) have been used to overcome the diffraction effects associated with conventional binary masks and to improve the exposure resolution and depth of images projected on a substrate. An attenuated PSM usually contains trasparent regions and partially transmissive regions. The partially transmissive regions provide a 180° phase shift and partial transmission, usually between about 3% and 10%, of the light they receive. The light passing through the partially transmissive regions (background light) destructivelly interferes with some of the light diffracted from the transparent regions of the attenuated PSM, thus reducing the detrimemtal effects caused by the diffracted light.
Using attenuated PSMs, on the other hand, may cause side lobe printing around the main patterns due to the non zero light transmission of the attenuated phase shift material, particularly in forming contact hole patterns. In side lobe printing, the background light and the diffraction light superimpose in the spaces between wanted features. In certain areas, the intensity of the superimposed lights is strong enough to cause a chemical reaction in the photoresist layer. The photoresist layer in these areas is then developed during the developing step, forming unwanted patterns (known in the art as side lobes) in the photoresist. The side lobes may be transferred to the substrate during subsequent fabrication processes and thus corrupt the desired features of the devices. The side lobe printing becomes more pronounced as the feature size becomes smaller and the spaces between the desired features decreases.
To prevent the problem of side lobe printing, prior art processes typically use masks with special built-in anti side lobe structures (see, for example, U.S. Pat. Nos. 5,700,606, 6,077,633 and 6,214,497). These processes are usually complicated and expensive. U.S. Pat. No. 6,465,160 discloses a method to mitigate side lobe printing by adjusting post-exposure baking time and temperature and soft-baking time and temperature to increase the contrast of the photoresist pattern and suppress the generation of side lobes. This method, however, may not be very effective.
Thus, there remains a need for an effective and convenient process to reduce side lobe printing in semiconductor device fabrication.